While this invention was developed for use in communicating data between various avionic systems and subsystems that need to share data, and is described in such an environment, it is to be understood that the invention can be utilized to communicate binary data in other environments. It is also to be understood that while the invention was developed for use with a current mode data bus, and is described in connection with such a bus, the invention can be utilized in connection with other types of data buses to improve the operation thereof, in particular, voltage mode and optical data buses. Similarly, while the invention was developed for use in a data communication system wherein the binary data to be communicated is in Manchester biphase form, it is to be understood that the invention can be used with binary data coded in other rectangular forms, such as binary data coded in mark-space form.
In modern aircraft, it is desirable to integrate, as far as possible, the functions of previous wiring-independent avionic systems to permit an attendant reduction in the weight, space and power requirements of the avionic systems, and to permit a simplification in wiring between physically separated avionic systems or subsystems thereof. Such integration has been achieved by the use of a common data bus to which each avionic system, or a subsystem thereof, has access through an associated terminal, each of which is capable of transmitting and receiving data. The data transmitted on the data bus by one terminal associated with a particular system or subsystem can be received by the terminals associated with remaining systems or subsystems, thus eliminating the requirement for separate wiring interconnections between the systems or subsystems. In addition, data generated by a particular system or subsystem can be used by any other system or subsystem without the necessity of having to independently generate that data.
While various types of communication systems that have been developed for use on-board aircraft to communicate between avionic systems and subsystems, as described in U.S. Pat. Nos. 4,199,663 and 4,471,481, both entitled "Autonomous Terminal Data Communications System" and assigned to the assignee of the present application, the most desirable avionic data communication system is an autonomous terminal data communication system, in particular, an autonomous terminal data communication system that uses a current mode data bus. Items critical to the operation of a data communication system that utilizes a current mode data bus are the reliability of the bus cable and the efficiency and reliability associated with the way each terminal is coupled to the bus. Current mode data bus coupling efficiency and reliability is addressed in U.S. Pat. No. 4,264,827 entitled "Current Mode Data Or Power Bus," also assigned to the assignee of the present application. The essence of the invention described in this patent is a coupling transformer having a ferrite core designed such that the core can be disassembled and two wires of a bus formed by a pair of twisted wires placed around the legs of the core in such a way that the magnetic path of the reassembled core surrounds the conductors. The arrangement is such that the bus wires form one of the windings of a transformer. The other winding is permanently installed on the core and is connected to the data transmitter and/or receiver electronics of a data terminal. The end result is the establishment of current coupling without the need to cut the bus wires or to remove or perforate the insulation that surrounds the wires.
Another item critical to the successful operation of a data communication system is the ability of the receiver electronics to accurately reproduce data signals carried by the data bus. If the signals are not accurately reproduced, they may be erroneously interpreted by utilization devices connected to the output receiver electronics. In this regard, data signals are frequently transmitted in rectangular form. An example of a rectangular data signal is a Manchester biphase data signal. Ideally, each transition of a rectangular wave data signal between signal levels is instantaneous. Unfortunately, the ideal does not exist. Rather, transitions between signal levels are exponential, with the time constant of the exponential transition being dependent upon the impedance characteristics of the data bus carrying the signal and the coupler that couples the data transmitter to the data bus. More specifically, as shown in FIG. 1, the bus signal, which represents transmitter generated Manchester biphase signals reproduced across the output terminals of a receive coupler transformer of the type described above, are not sharp. Rather, the transitions exponentially change over a discrete period of time.
In the past, bus signals of the type illustrated in FIG. 1 have been detected by a pair of oppositely biased comparators. The bias level is shown by the dashed lines located above and below the zero signal line of the bus signal section of FIG. 1. The outputs of the receiver comparator are shown on the receiver comparator lines of FIG. 1. While the mark-space transitions of the outputs of the receiver comparators are sharp, the comparator outputs are not an accurate reproduction of the originally transmitted Manchester biphase signal. Rather than being complementary, a gap exists between transitions of the comparator outputs. That is, both comparator outputs are at a common low level for a short period of time between the high-low transition of one comparator and the low-high transition of the other comparator. The length of the gap is related to the exponential transition time of the bus signal. This gap is commonly eliminated by downstream reconstruction circuits that respond only to the leading edges of the comparator output signals. As a result, the high-low transition of one biphase signal does not occur until the low-high transition of the other biphase signal. The result of this reconstruction is shown in the last lines of FIG. 1, which also illustrates the disadvantage of this approach, namely that the duty cycle of the first cycle of the data signal, in this case the sync pattern of the Manchester biphase signal, is not fifty percent. Rather, the first half of the mark-space cycle, denoted M1, is greater than the second half, denoted N1. As a result, the receive coupler reconstructed Manchester biphase signal, RXI and RXN, does not accurately represent the transmitter generated Manchester biphase signal, TX0 and TXN. This difference can lead to an erroneous interpretation of the output of the receive coupler by the utilization device to which it is connected. The invention is directed to providing a receive coupler that overcomes this problem.